Nuclear radiation gauges have been widely used for measuring the density and moisture of soil and asphaltic materials, or other construction material. As used herein, construction material is any materials used in building roads or foundational structures including, but not limited to soils, asphalts, asphalt-like materials, concrete, composite materials, or the like. Such gauges typically include a source of gamma radiation which directs gamma radiation into the test material, and a radiation detector located adjacent to the surface of the test material for detecting radiation scattered back to the surface. From this detector reading, a determination of the moisture and density of the material can be made.
These gauges are generally designed to operate either in a “backscatter” mode or in both a backscatter mode and direct transmission mode. In gauges capable of direct transmission mode, the radiation source is vertically moveable from a backscatter position, where it resides within the gauge housing, to a series of direct transmission positions, where it is inserted into small holes or bores in the test specimen.
Many of the gauges commonly in use for measuring density of soil, asphalt and other materials are most effective in measuring densities of materials over depths of approximately 3-12 inches. However, with the increase in cost of paving materials, the practice in maintaining and resurfacing paved roadbeds has become one of applying relatively thin layers or overlays having a thickness of one to three inches. With layers of such a thickness range, many density gauges are ineffective for measuring the density of the overlay because the density reading obtained from such gauges reflects not only the density of the thin layer, but also the density of the underlying base material.
Nuclear gauges capable of measuring the density of thin layers of materials have been developed by Troxler Electronic Laboratories, Inc. of Research Triangle Park, N.C. For example, thin layer density gauges are disclosed in U.S. Pat. Nos. 4,525,854, 4,701,868, 4,641,030, 6,310,936 and 6,442,232, all of which are incorporated herein by reference in their entirety. Some of the gauges disclosed in the above-referenced patents are referred to as “backscatter” gauges because the radiation source does not move outside the gauge housing, which is necessary for measurement in the direct transmission mode. In some of the gauges disclosed in the above-referenced patents, the gauge can have radiation sources that can also be extended outside of the gauge housing and into the material to be measured in a direct transmission mode. Typically, the source rods can extend up to about 12 inches.
As disclosed in the above patents, the preferred method of measuring the density of thin layers of materials, such as asphalt, is nondestructive and uses the backscatter mode. One method requires two independent density measurement systems. The geometry of these two measurement systems must be configured with respect to one another and with respect to the medium being measured in such a manner that they measure two different volumes of material. The two different volumes are not mutually exclusive insofar as they partially overlap one another. Measurement accuracy depends upon a larger portion of the volume measured by one of the measurement systems being distributed at a lower depth beneath the gauge than the volume measured by the other measurement system. This is accomplished by placing one radiation detection system in closer spatial proximity to the radiation source than the other detection system. Another volume specific measurement is typically used in soils and requires drilling a small hole in the material under test. This method is referred to as the direct transmission mode.
To determine the positioning of the source rod during use normally includes a visual inspection of the location of the source rod relative to an index rod and/or the height of the portion of the source rod extending out of the gauge housing. Such determination can be problematic and inaccurate. Contact strips whose resistance varies with position have also been used to detect the length that the source rod has moved. These strips often wear out.
Preparation for configuring a gauge can be time consuming. For gauges used in the past, each type of gauge would be configured differently so that there would be multiple configuration programs for gauges. Thus, each type of gauge could have a separate configuration program written for it.
Also, as known in the art, the calibration of a nuclear gauge, for example, a 12-position nuclear gauge is time consuming, and many quality control checks have to be implemented. For instance, programs over the years have been developed that analyze the calibration curves to find statistical variations in the gauge. For example, the typical calibration constants, count rate, precision and slope as a function of density of each gauge, along with their standard deviations, have been determined. These parameters are an important part of the diagnostics of the health of a gauge. Currently, only at the factory can this sort of diagnostics be accomplished. In the factory, external computer networks are wired to each calibration bay, the data is transferred by wire from the instrument to the external computer, where computer programs known in the art are used to curve fit, transfer the coefficients, store the coefficients to the gauge, and quality control check each measurement for deviations out of the standard expected values.
There remains a need in the art for a nuclear gauge capable of operating in backscatter mode and/or direct transmission mode, and which is suitable for efficiently measuring the density and moisture of construction material.